Titanium dioxide (TiO2) is commercially produced by either the sulfate process or the chloride process. In the chloride process, titania-containing feedstocks are chlorinated to form titanium tetrachloride which is then oxidized to form TiO2. This process operates most efficiently starting from titania-containing feedstocks having high TiO2 content. Due to steady growth in the TiO2 markets, existing supplies of titania-containing feedstocks, such as ilmenite and natural rutiles, are coming under pressure. The new beach sand/placer deposits are of variable quality and many are unsuitable for upgrading and beneficiation using existing commercial processes.
Various titaniferous ores have high concentrations of zircon and monazite minerals due to their geological proximity. The zircon and monazite impurities in the feedstock reduce its market value. Actinide and lanthanide impurities create operational problems (e.g., high chlorine consumption or sticky beds) and generate hazardous waste with high concentrations of actinides, lanthanides and other heavy metals. Due to stringent environmental regulations in many countries, the treatment and disposal of such hazardous waste from chloride and sulfate process plants has resulted in increased cost of waste treatment and management.
Many different beneficiation methods for improving the TiO2 content of titaniferous ores have been developed. Conventional processes include physical processes such as gravitational, magnetic and electrical separation which are used to separate the magnetite, monazite, zircon and other siliceous gangue. Other conventional processes are chemical processes, such acid leaching and TiO2-slag formation (high temperature reduction), such as the Becher process, which are mainly used to remove iron.
Unfortunately, these conventional processes require high quality ilmenite ores. The ores containing monazite, zircon and actinides are not reduced even at high temperatures because of the bound heavy metals as phosphates. The level of critical impurities such as Cr2O3, V2O5, Nb2O5 (which degrade pigment properties) and CaO, Al2O3 and SiO2 (which create operational problems such as sticky beds) remains very high in the end product. Also, solute impurities (Fe, Nb, U, Th, Ce) in the TiO2 phases (pseudorutile, ilmenite, anatase) remain in the feedstock and end up in the waste stream of the pigment-grade TiO2 manufacturing process (chlorination or sulphatation).
The slagging process (which is the main source of feedstock for pigment-grade TiO2 manufacturing) separates only iron oxides and most of the other impurities remain in the feedstock. The slagging process also faces uncertainty due to its high power consumption and emission of greenhouse gases during electric arc smelting.
In view of the changing sources (deposits) of TiO2 ores and environmental concerns in relation with the disposal of waste, there is a need for a more environmentally acceptable approach to the beneficiation of titaniferous ores. The roasting of ilmenite with soda, mainly in a reducing atmosphere with carbon has been disclosed (El-Tawil et al, Alkali reductive roasting of ilmenite ore, Canadian Metallurgical Quarterly, 1996, 35(1), 31-37). However, the yield of TiO2 in this technique is not very high (<90 wt. %) and iron is neither separated in the metallic form nor is a leachable product produced. U.S. Pat. No. 6,346,223 teaches oxidative alkali roasting techniques. However, the yield of TiO2 and the separation of actinides and lanthanides is below the required levels for the chloride process.
In sum, there is a need to develop processes for beneficiating titaniferous ores to produce beneficiated ores having high titania content and low impurity levels. The present invention provides such an improved process.